Measurements and Evaluation of Flicker in High Voltage

International Conference on Renewable Energies and Power Quality (ICREPQ‟14)

Cordoba (Spain), 8th to 10th April, 2014 Renewable Energy and Power Quality Journal (RE&PQJ)

ISSN 2172-038 X, No.12, April 2014

Measurements and Evaluation of Flicker in High Voltage Networks

M. H. Albadi

1, A. S. Al Hinai

1, A. H. Al-Badi

1, M. S. Al Riyami

2, S. M. Al Hinai

1, and R. S Al Abri

1

1 Department of Electrical and Computer Engineering

Sultan Qaboos University (SQU)

Muscat-123, Sultanate of Oman

Tel: +968 2414 2664, Fax: +968 2441 3454

Email: [email protected], [email protected], [email protected], [email protected], [email protected] 2 Oman Electricity Transmission Company (OETC)

Tel: +96824497279, Fax: +968 24497748, Email: [email protected]

Abstract. This paper presents a case study about the impacts

of industrial loads on power quality. A review of flicker causes,

negative impacts, mitigation techniques, quantification indices

and limits are presented. Moreover, flicker measurements at three

grid station that supply the three main industrial areas located in

the main interconnected system (MIS) of Oman are conducted.

Measurement results are compared with limits specified by

national and international standards.

Key words Power Quality, Flicker, Measurements, Standards, Arc

furnace.

1. Introduction Heavy industrial loads are often connected to transmission

networks at high voltage levels such as 132 kV and 220

kV. These loads may include rolling mills often driven by

thyristor-control DC Drives, or could be an Electric Arc

furnace or induction furnaces needing large reactive

power. These types of loads can deteriorate the supply

quality of other loads connected to the same point of

common coupling (PCC).

To study the impact of heavy industrial customers on the

power quality of the high voltage network, measurement

of the existing distortion levels is important. In this paper,

flicker measurements are conducted at selected industrial

load grid stations of Oman Electricity Transmission

Company (OETC) network [1]. The on-site measurements

are performed at the 132kV side of the substation

transformer feeding the industrial customers. The

measured levels of flicker distortion are analyzed and

compared with appropriate standards. The study outcome

and recommendations can be used to further improve the

grid code and connection agreements for heavy industries.

After this introduction, the paper proceeds with presenting

an overview of the voltage flicker, its impacts, typical

sources and mitigation techniques. In section three, flicker

indices and limits are reviewed. Measurement results are

discussed in section four. Finally the main conclusions are

presented.

2. Flicker Flicker is simply the fluctuation of light intensity.

According to the International Electro technical

Vocabulary (IEV), it is defined as: “Impression of

unsteadiness of visual sensation induced by a light

stimulus whose luminance or spectral fluctuate with

time” [2]. In many cases, the term „Flicker‟ is used to

describe the voltage fluctuations themselves.

Since the inception of electric lighting, flickering of

lights has been a reality for most consumers. Flicker is

relatively an old topic. According to [3], it was the flicker

that determined the standard frequency for the electric

power system to prevent visible flickering in open-type

arc lamps in America in 1891 to be 60Hz. Europeans

select 50 Hz based on the flicker response of enclosed-

type arc lamps [3].

A. Effect of Flicker

Light flicker is just a symptom perceived by the human

eye of the original problem, which is voltage fluctuation

caused by rapid variation of the load current. If voltage

fluctuations occur with certain frequencies (0.05-35 Hz),

the fluctuation in light intensity, perceived by the human

eye, could lead to unpleasant feeling and health

problems. The most severe disturbing effect occurs when

the frequency of the fluctuation is 8.8 Hz. The flicker

phenomena can be so subtle as to not be consciously

detected by those affected but at the same time can

induce discomfort in the form of nausea or headache and

in turn affects the work efficiency. Normally flicker is

perceived as annoyance and distraction. But in some

extreme cases, it can cause epilepsy seizures to those who

are borne to it. It has been reported that flicker causes

seizures in about 4% of the patients with epilepsy [4].

Voltage fluctuations can cause other problems other than

light flickers. These include triggering of UPS units to

switch to battery and problems with some sensitive

electronic equipment, such as in medical laboratories,

which require a constant voltage. When the voltage

magnitude varies, power flow into the equipment will

also change. If the variation is large enough or within a

critical frequency range, the performance of equipment

can be affected [5]. Usually, complaints about light

flicker will occur well before any disturbances on other

loads show up [6]. As an example to this fact, the ITI

(CBEMA) curve shows the faultless operation zone of

the IT equipment as a function of nominal voltage change

https://doi.org/10.24084/repqj12.218 38 RE&PQJ, Vol.1, No.12, April 2014

mailto:[email protected]






and the duration of that change. This curve describes an

AC input voltage envelope which typically can be

tolerated by most Information Technology Equipment

(ITE) [7].

Fig. 1. Faultless operation zone of the IT equipment (ITI curve)

B. Causes of Flicker

In principle, any switching operations of industrial

processes and electrical appliances connected to the supply

system can cause voltage changes and in turn light flicker.

Generally, flicker phenomenon can be divided into two

general categories, cyclic and non-cyclic. Cyclic flicker is

caused by periodic load fluctuation/switching operations.

Non-cyclic flicker is caused by occasional load

fluctuations/switching operations. Some sources of flicker

can cause both cyclic and non-cyclic flicker. The typical

sources of light flicker are listed below [5].

- Arc furnaces cause fast voltage fluctuation occurs due

to the high current and the unstable nature of the arc

during the process.

- Resistive welding machines draw high repetitive

welding currents that cause the voltage to fluctuate in

the same manner.

- Large electric motors can cause flickers during

starting due to the inrush starting current and if the

load is varying. Examples of such motors are rolling

mills, stone crushers, elevators and pumps.

- on/off switching of large loads such as large capacity

copy machines, medical imaging machines and color

organs in discos.

- on/off switching of compensating elements and

changing of transformer taps cause a sudden

increase/decrease of the voltage.

- The operation of intermittent types of distributed

generation facilities such as wind turbines and PV

systems.

- Pulsated loads such as Harmonics active filters and

Thermostat controlled loads.

- System faults in electrical supply system can cause

temporary voltage fluctuations. These kinds of events

may cause very high flicker levels but are not

frequent.

C. Flicker Mitigation

Flicker occurs due to two reasons: network impedance

and fluctuated load current. To minimize flicker, the

magnitude of voltage fluctuations must be reduced. To

achieve this, two strategies can be used: a) Reduce the

network impedance. b) Reduce the load current

fluctuation.

The power utility is responsible for the first option. This

can be achieved through two approaches:

1. Network Reinforcement:

Reducing network impedance means increasing the short

circuit power at the load bus. This is achieved by

reducing the impedances between the load and the supply

through network reinforcement. Unfortunately, this

solution is economically not justifiable.

2. System Reconfiguration:

A cheaper solution could be to increase the short circuit

power at the PCC with the existing system infrastructure.

On the other hand, reducing the current fluctuation of the

load should be done by the load (network customer). This

can be achieved through reactive compensation. The

voltage drop caused by a variable/fluctuating load can be

decomposed into two components: reactive component

and active component. The reactive load component is

dominant due to high X/R ratio. There are different

options for reactive compensation: Shunt Capacitors,

Synchronous Condenser, Static Var Compensation

(SVC), and Static synchronous compensation

(STATCOM).

3. Flicker Indices and Limits A. Flicker Indices

The flicker assessment is basically based on human

perception of voltage fluctuations with certain outward

shapes and various repetition rates (frequencies). The

international Electrotechnical Commission standard IEC

61000 4-15 describes the flicker measuring

methodology[8]. Two indicators are used to assess the

flicker severity [8, 9].

- Pst (P for Perceptibility) is the short term flicker

severity index evaluated over a short period. The

standardization bodies specify the time over which

the Pst is calculated to be 10 minutes [2, 8, 9].

- Plt is the long term flicker severity index evaluated

over a long period. It calculated for 2 hours of the

observation period using 12 pieces of successive 10

minutes Pst values [2, 8, 9].

3

12

1

3

12

1

j

stlt PP (1)

When there are more than one flicker source, the total

flicker severity can be determined by adding the

annoying factors caused by individual sources.

33

i

istst PP (2)


At Pst =1 p.u flicker severity, 50% of the people exposed

to flicker will feel unpleasant or irritable. This flicker level

is called “the threshold of irritability”. The international

standardization bodies (IEC & IEEE) provide tables and

curves showing Pst=1 as a function of the percentage

relative voltage change and the frequency of the voltage

changes [9].

The international standardization bodies (IEC & IEEE)

provide tables and curves showing Pst=1 as a function of

the percentage relative voltage change and the frequency

of the voltage changes [9]. An example is shown in the

following figure.

Fig. 2. Pst = 1 test points for rectangular voltage fluctuations [9]

Figure 2 shows the Pst =1 curve of a 60W (230V and

120V) incandescent lamp for a rectangular voltage change

according to IEC and IEEE [9]. It can be seen that the

Pst=1 curve have a minimum relative voltage changes

value of 0.29% at 1052 changes per minute i.e. 8.8Hz .

Note that 1Hz voltage fluctuation means 120 voltage

changes per minute.

The relative voltage change should be always in the area

below the Pst=1 curve to be admissible. However, at low

changing rates below 0.6 changes per minute, the threshold

level of Pst=1 is not valid as a voltage fluctuations limit

because the corresponding (d %) is too high. An example

for this case is an occasional large voltage change. This

voltage change has small effect on the Pst value but it will

be very disturbing to other loads connected to the same

point of common coupling (PCC). This example could be

an appliance that draws a very large inrush current from

the supply when it is first switched on, especially if the

input has no inrush current limiting circuitry. The inrush

current may cause a substantial deviation in the supply

voltage causing disturbance to sensitive loads connected to

the same PCC.

B. Flicker Limits

IEC 61000 3-7 provides guidance to system operators,

owners and engineers that are responsible for providing

electrical service to loads that cause voltage fluctuations.

The relevant IEC standard has been recently adopted by

IEEE [10, 11]. It is worth mentioning that compatibility

levels are not defined by IEC 61000 3-7 for MV, HV and

EHV systems. However, indicative values of planning

levels are presented as shown in the following table.

Table I IEC indicative values of planning levels for flicker [11]

Indicator MV

1 kV < U ≤ 35 kV

HV & EHV

U> 35 kV

Pst

Plt

0.9

0.7

0.8

0.6

According to the national grid code [12], “the level of

voltage fluctuations at a Connection Point shall be within

the limits defined in IEC 61000-3-7, with a Flicker

Severity (Short Term) of 0.8 Unit and Flicker Severity

(Long Term) of 0.6 Unit.”

4. Measurement Results and Discussion

There are three main industrial areas in the MIS of

Oman: Rusail area in Muscat, Sohar Industrial Area and

Nizwa industrial area. Data of relevant grid station are

listed in Table II.

Table II Grid stations data [1]

Grid

Station

Number of

Tx

Rating of

each Tx

Max short circuit

level on 132 kV

side (kA)

Rusail 4 132/33

kV 75 MVA 17.76

Sohar

SIA

2

220/132

kV

500 MAV 16.77

Nizwa 2 132/33

kV 125 MVA 14.05

Measurements were conducted using Hioki 3196 Power

Quality Analyzer Meter [13]. 10-minute average values

were recoded over a period of 1 week in each location.

A. Rusail Grid Station

A summary of flicker measurements at Rusail grid station

is presented in Figure 3. A and B in the figure show the

start and the end of the measurement period.

As seen from Figure 3, the average value of the short

term flicker severity index (Pst) ranges between 0.262

and 0.463. The maximum value of Pst is 0.811. This

value is slightly higher than the limit specified by the grid

code (0.8).

For the long term flicker severity index (Plt), the average

value ranges between 0.265 and 0.464. The maximum

value of Plt reaches 0.522, which is lower than the limit

specified by the grid code (0.6). A sample of the

chronological variation of flicker level at Rusail grid

station is shown in Figure 4.

For the long term flicker severity index (Plt), the average

value ranges between 0.265 and 0.464. The maximum

value of Plt reaches 0.522, which is lower than the limit

specified by the grid code (0.6). A sample of the

chronological variation of flicker level at Rusail grid

station is shown in Figure 4.

0.1

1

10

0.01 0.1 1 10 100 1000 10000

Rela

tive v

olt

ag

e c

ha

ng

e (

d%

)

Rectangular voltage fluctuations per minute

Pst=1 for 230 V lamps

Pst=1 for 120 V lamps


Fig. 3. Summary of Flicker measurements at Rusail grid station

Period: 2012/01/17 11:41:00 - 2012/01/24 11:41:00

Fig. 4. Flicker measurements at Rusail grid station.

B. Sohar SIA Grid Station

The power quality analyzer was set to record required data

for a period of one week starting from 28th of January

2012. A summary of flicker measurements at Sohar SIA

grid station is presented in Figure 5.

The average value of the short term flicker severity index

(Pst) are 0.737, 0.839 and 0.835. The last two average

values are higher than the limit specified by the grid code

(0.8). The highest recorded Pst value is 2.148.

For the long term flicker severity index (Plt), the

recorded values range between 0.191 and 1.193. All

average values are higher than the 0.6 limit specified by

the grid code.

Fig. 5. Summary of Flicker measurements at Sohar SIA grid

station

A sample of the chronological variation of flicker level at

Sohar SIA grid station is shown in Figure 6.

Fig. 6. Flicker measurements at Sohar SIA grid station

This high level of flicker distortion is attributable to the

operation of arc furnaces. A simultaneous measurement

of the current in the 132 kV feeder supplying the arc

furnace demonstrates the relationship between the

operation of arc furnaces and the high level of flicker as

demonstrated in Figure 7.

01/17

11:51:00

01/18

07:51:00

01/19

03:51:00

01/19

23:51:00

01/20

19:51:00

01/21

15:51:00

01/22

11:51:00

01/23

07:51:00

01/24

03:51:00

01/24

23:51:00

01/25

19:51:00

1.200

0.800

0.400

0.200 /div Pst CH3

01/17

11:51:00

01/18

07:51:00

01/19

03:51:00

01/19

23:51:00

01/20

19:51:00

01/21

15:51:00

01/22

11:51:00

01/23

07:51:00

01/24

03:51:00

01/24

23:51:00

01/25

19:51:00

1.200

0.800

0.400

0.200 /div Plt CH3

01/28

13:14:00

01/29

09:14:00

01/30

05:14:00

01/31

01:14:00

01/31

21:14:00

02/01

17:14:00

02/02

13:14:00

02/03

09:14:00

02/04

05:14:00

02/05

01:14:00

02/05

21:14:00

3.000

2.000

1.000

0.000

0.500 /div Pst CH3

01/28

13:14:00

01/29

09:14:00

01/30

05:14:00

01/31

01:14:00

01/31

21:14:00

02/01

17:14:00

02/02

13:14:00

02/03

09:14:00

02/04

05:14:00

02/05

01:14:00

02/05

21:14:00

1.200

0.800

0.400

0.200 /div Plt CH3


Fig. 7. Current measurements of the 132kV feeder connected to

an arc furnace at Sohar SIA grid station

C. Nizwa Grid Station

In Nizwa grid station, the analyzer was set to record

required data for a period of one week starting from 29th

of February 2012. As demonstrated in Figure 8, the

average values of the short term flicker severity index for

the three phases are 0.661, 0.621 and 0.6. The highest

recorded Pst value is 2.695.

Fig. 8. Summary of Flicker measurements at Nizwa grid station

It is worth mentioning that the Pst value is lower than the

limit specified by the grid code (0.8) most of the time.

However, this is not the case for the recorded Plt values. In

most of the cases, the Plt values were higher than the 0.6

limit specified by the grid code. The recorded values range

between 0.545 and 1.25. All average recorded values are

higher than the 0.6 limit.

A sample of the chronological variation of flicker level at

Nizwa grid station is shown in Figure 9.

Spikes in the flicker measurements might be attributable to

the unstable 132kV single circuit connection with

Petroleum Development Oman (PDO) power network. The

tripping of this connection causes voltage spikes as seen

in Figure 10.

Fig. 9. Flicker measurements at Sohar SIA grid station

Fig. 10. Voltage measurements at the 132kV side of Nizwa grid

station

5. Conclusions This paper presents a review of flicker causes, impacts,

mitigation techniques, indices and limits. Moreover, a

case study about flicker measurements at three grid

stations feeding industrial areas in the main

interconnected system of Oman is presented. The

measured flicker levels in two of the three grid stations

violate the limits specified by the national grid code. It is

worth mentioning that Sohar SIA grid station has the

highest flicker distortion level due to the operation of arc

furnaces. It is recommended to include limits on flicker

levels emitted from a single industrial customer in the

grid code. As heavy industry loads are expanding in the

country, it is expected that power quality distortion levels

will increase. Therefore, continuous monitoring of power

quality distortion levels is important.

0.0500kA/ div CH3 AVE

0.3000k

0.2000k

0.1000k

0.0000k

01/ 28

13:14:00

01/ 29

09:14:00

01/ 30

05:14:00

01/ 31

01:14:00

01/ 31

21:14:00

02/ 01

17:14:00

02/ 02

13:14:00

02/ 03

09:14:00

02/ 04

05:14:00

02/ 05

01:14:00

02/ 05

21:14:00

02/ 06

17:14:00

02/ 07

13:14:00

02/ 08

09:14:00

02/29

12:21:00

03/01

08:21:00

03/02

04:21:00

03/03

00:21:00

03/03

20:21:00

03/04

16:21:00

03/05

12:21:00

03/06

08:21:00

03/07

04:21:00

03/08

00:21:00

03/08

20:21:00

1.600

1.200

0.800

0.200 /div Pst CH3

02/29

12:21:00

03/01

08:21:00

03/02

04:21:00

03/03

00:21:00

03/03

20:21:00

03/04

16:21:00

03/05

12:21:00

03/06

08:21:00

03/07

04:21:00

03/08

00:21:00

03/08

20:21:00

1.600

1.200

0.800

0.200 /div Plt CH3

4.00kV/ div U3

80.00k

72.00k

64.00k

02/ 29

12:21:00

03/ 01

08:21:00

03/ 02

04:21:00

03/ 03

00:21:00

03/ 03

20:21:00

03/ 04

16:21:00

03/ 05

12:21:00

03/ 06

08:21:00

03/ 07

04:21:00

03/ 08

00:21:00

03/ 08

20:21:00

03/ 09

16:21:00

03/ 10

12:21:00

03/ 11

08:21:00


ACKNOWLEDGMENTS

The authors would like to acknowledge Oman Electricity

Transmission Company (OETC) and Sultan Qaboos

University (SQU) for the support in achieving this work.

This study is financially supported by OETC under project

number CR/ENG/ECED/11/06.

6. References [1] Oman Electricity Transmission Company (OETC) website,

http://www.omangrid.com/.

[2] International Electrotechnical Vocabulary- Chapter 161:

Electromagnetic Compatibility (IEV: 161-08-13), IEC 60050 part

161,

http://www.electropedia.org/iev/iev.nsf/display?openform&ievref

=161-08-13.

[3] (1996) Flicker still a concern after all these years. System

Compatibility Research News.

[4] P. M. Jeavons and G. F. A. Harding, Photosensitive Epilepsy:

A Review of the Literature and a Study of 460 patients. London:

William Heinemann Books, 1972.

[5] M. M. Morcos and J. C. Gomez, "Flicker Sources and

Mitigation," IEEE Power Engineering Review, vol. 22, pp. 5-10,

2002.

[6] J. Schlabach, D. Bulme, and T. Stephanblome, Voltage

Quality in Electrical Power Systems. UK- London: Institute of

Electrical Engineers (IEE), 2001.

[7] ITI (CBEMA) Curve, Information Technology Industry

Council (ITI), http://www.itic.org

[8] IEC, "IEC 61000-4-15, Electromagnetic compatibility (EMC)

– Part 4: Testing and measurement techniques – Section 15:

Flickermeter – Functional and design specifications, 2010.

[9] IEEE Recommended Practice--Adoption of IEC 61000-4-

15:2010, Electromagnetic compatibility (EMC)--Testing and

measurement techniques--Flickermeter--Functional and design

specifications," in IEEE Std 1453-2011, pp. 1-58.

[10] IEEE Draft Adoption of IEC/TR 61000-3-7:2008,

Electromagnetic compatibility (EMC)-Limits-Assessment of

emission limits for the connection of fluctuating installations to

MV, HV and EHV power systems, in IEEE Std P1453.1/D2,

March 2012, pp. 1-78.

[11] IEC, IEC 61000-3-7, Electromagnetic compatibility (EMC)-

Limits-Assessment of emission limits for the connection of

fluctuating installations to MV, HV and EHV power systems,

2008.

[12] OETC, "The Grid Code, Version 2," ed, 2010,

http://www.omangrid.com/en/Grid%20Code.aspx.

[13] HIOKI E.E. CORPORATION, Hioki 3196 Power Quality

Analyzer Meter, http://www.hioki.com/product/3196/3196v.html.

7. Biographies Mohammed H. Albadi received the B.Sc. degree in

electrical and computer engineering from Sultan Qaboos

University, Muscat, Oman in 2000; the M.Sc. degree in

electrical engineering from Aachen University of

Technology, Germany in 2003; the Ph.D. degree in

Electrical and Computer Engineering from University of

Waterloo, Canada in 2010. He is currently working as

Assistant Professor in the Electrical & Computer

Engineering Department at Sultan Qaboos University,

Muscat, Oman. His research interests include Renewable

energy, Distributed Generation, Power Quality,

Distribution systems, Demand side management, Power

system operation and planning, and Power system

economics.

Amer. S. Al Hinai received his M.Sc. and PhD in

Electrical Engineering from West Virginia University,

Morgantown, USA in December 2000 and May 2005,

respectively. He is an Assistant Professor in the Electrical

& Computer Engineering Department at Sultan Qaboos

University and a visiting professor at Masdar Institute of

Science & Technology, Abu Dhabi, UAE. He is holding

a member position at the Authority of Electricity

Regulation – Oman. His research interests are related to

energy savings, power system analysis, power system

quality and transient stability of power systems.

Abdullah H. Al-Badi obtained the degree of B.Sc. in

Electrical Engineering from Sultan Qaboos University,

Oman, in 1991. He received the degree of M.Sc. and

Ph.D from UMIST, UK, in 1993 and 1998 respectively.

In September 1991, he joined the Sultan Qaboos

University as demonstrator and, in 1998, he became an

Assistant Professor. Currently he is Professor at the

department of electrical and computer engineering and

the Dean of Admission & Registration at Sultan Qaboos

University. His research interests include electrical

machines, drives, AC interference and renewable energy.

Masoud S. Al Riyami obtained his degree from Sultan

Qaboos University in 1994 in Electrical and Electronics,

major power systems. After graduation, he worked for

the Ministry of Housing, Electricity and Water. After the

restructuring of the electricity sector in Oman in 2009, he

moved to Oman Electricity Transmission Company

S.A.O.C (OETC). He is currently working as a Senior

Manager for Transmission Department. He is serving as a

member in many internal committees in OETC as well as

the chairman of product approval committee in OETC.

Salem M. Al-Hinai received the BSc. in Electrical

Engineering from Sultan Qaboos University, Muscat,

Oman in 2001 and joined the University as a

demonstrator in 2001. He has worked as Teaching

Assistant in ECE Department at Dalhousie University.

Currently, he is Lecturer in the Electrical & Computer

Engineering Department at Sultan Qaboos University.

His research interests are in modelling, operation,

protection and control of power systems and distributed

generators.

Rashid S. Al-Abri was born in Oman. He received the

B.Sc in electrical engineering from Sultan Qaboos

University, Oman, in 2002. Then he did his M.Sc. in

electrical engineering from Curtin University of

Technology, Western Australia, and Ph.D. degree from

the Department of Electrical and Computer Engineering,

University of Waterloo, Waterloo, ON, Canada.

Currently, he is with Sultan Qaboos university, Muscat,

Oman. His research interests are power system stability,

power quality, power electronics, and distributed

generation.


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Measurements and Evaluation of Flicker in High Voltage